hormones 2

• Endocrinology is a branch of medicine concerned with the structure, function, and disorders of the endocrine glands
• Endocrine functions are necessary to:

  • Maintain homeostasis (insulin and glucagon maintain blood glucose levels)
  • Regulate growth, development, differentiation, and reproduction (menstrual cycle, pregnancy)
  • Respond to external stimuli (fight or flight response by stress hormones)
• The coordination of metabolism in mammals is achieved by the neuroendocrine system, which involves the nervous system and the endocrine system
• Communication Molecules:

  • Neurotransmitters in the nervous system
  • Hormones in the endocrine system
• In the nervous system:

  • Electrical signals (nerve impulses) originate in the neuron and travel rapidly to the axon tip where neurotransmitters are released
  • The chemical messenger (neurotransmitter) may travel only a short distance across the synaptic cleft to the next neuron
• In the endocrine system:

  • Hormones are secreted into the bloodstream and carried to target tissues that are away from the secreting cell
• Hormones are chemical messengers produced in small amounts and have physiological effects at low concentrations
• Receptors:

  • Receptors are cellular proteins that bind hormones
  • Binding of ligands to receptors is never covalent, always reversible, specific, and saturable
• Endocrine hormones are released into the blood and carried to target cells throughout the body (e.g., insulin)

  • Paracrine hormones are released into the extracellular space and diffuse to neighboring target cells (e.g., eicosanoid hormones)
  • Autocrine hormones are released by and affect the same cell, binding to receptors on the cell surface (interleukin)
• Hormones are synthesized in endocrine glands designed for specific purposes

  • An endocrine gland is a group of specialized cells that synthesize and secrete hormones
• Major Endocrine Systems and Their Target Tissues:

  • Hypothalamic hormones are secreted into hypothalamo-hypophyseal portal veins and enter adenohypophysis (anterior pituitary)
• Hormone Classification:

  • Mechanism of action
  • Chemical structure
  • Site of synthesis (endocrine organ)
  • Function
  • Mode of storage and secretion
• Group I Hormones:

  • Steroid hormones
  • Thyroid hormones
  • Vitamin D
  • Retinoids
• Steroid Hormones:

  • Control metabolism, inflammation, immune functions, salt and water balance, development of sexual characteristics, and the ability to withstand illness and injury
  • Synthesized from cholesterol in the gonads and adrenal glands
• Thyroid Hormones:

  • Thyroglobulin is the precursor of thyroid hormones
  • Stimulate energy-yielding metabolism, especially in the liver and muscle
• Vitamin D Hormone (Calcitriol):

  • Important for calcium and phosphorus absorption in the intestines
  • Regulates calcium and phosphorus levels in the blood
• In multicellular organisms, the distance for substances to enter cells is larger due to a higher surface area to volume ratio
• Thyroid hormones stimulate energy-yielding metabolism, especially in the liver and muscle
• Thyroid hormones increase the expression of genes encoding key catabolic enzymes
• Calcitriol is the active form of vitamin D obtained through diet or sunlight exposure
• Vitamin D is involved in mineral metabolism and bone growth
• Vitamin D facilitates intestinal absorption of calcium, phosphate, and magnesium ions
• Retinoids regulate the growth, survival, and differentiation of cells
• Retinol is synthesized from vitamin A, primarily in the liver
• Retinol is converted to the hormone retinoic acid (RA) in many tissues
• Ribosomal synthesis of polypeptide hormones occurs on rough ER ribosomes
• Enzymatic synthesis involves hormones being synthesized from precursor molecules through enzymatic modifications
• Thyroid hormone synthesis involves ribosomal synthesis of thyroglobulin and enzymatic activation of iodide
• Thyroid hormone synthesis includes the incorporation of iodine to tyrosine residues in thyroglobulin
• Thyroid hormone synthesis involves the coupling of iodinated tyrosines
• Thyroid hormone synthesis includes the enzymatic hydrolysis of thyroglobulin to release coupled tyrosines
• Hormone synthesis and secretion are regulated by endocrine and neuroendocrine control mechanisms
• Regulation mechanisms include releasing factors, stimulating hormones, positive feedback, and negative feedback
• Hormones are classified based on solubility, binding proteins, plasma half-life, receptors, and response time
• Amino acid derivative hormones include catecholamines, serotonin, melatonin, and thyroid hormones
• Hormones like dopamine, adrenaline, and noradrenaline are synthesized from tyrosine amino acid
• Hormones like histamine, serotonin, and GABA are derived from specific amino acids
• Specific hormones are synthesized in different body organs like the hypothalamus, pituitary, thyroid, parathyroid, pancreas, adrenal glands, ovaries, testes, gastrointestinal system, skin, liver, and kidney
• Hormones regulate various functions like glucose metabolism, calcium and phosphate metabolism, water and salt metabolism, anabolic processes, catabolic processes, and stress responses
• Storage and secretion of hormones vary, with some stored as prohormones, in intracellular vesicles, or not stored at all